Two-beam interferometer spectrometers are well known in the art. Most of the modern spectrometer designs, including the Michelson interferometer spectrometer, are described in Griffiths and DeHaseth, Fourier Transform Infrared Spectrometer, John Wiley & Sons, N.Y., Chapter 4, 121-165 (1986).
The Michelson interferometer functions by dividing an incident beam of electromagnetic radiation into two separate beams at a beamsplitter, each of which beams is caused to travel along a different optical path; one of the beams is usually directed along a reference path having a fixed optical length, while the other is simultaneously directed along a path of variable length. A retro-reflecting element, such as a plane mirror, a cube corner, or a "cats eye" reflector, is placed in the optical path of each beam to return it to the beamsplitter, where the beams recombine to form a single exit beam. The optical path length variation, which is typically achieved by moving one of the retroreflectors, causes the combined exit beam to be amplitude modulated due to interference between the recombining beams. By converting the interferogram so obtained into an electrical signal, and by then analyzing that signal in a known manner, the spectrum or intensity of the input radiation can, after suitable calibration, be derived as a function of frequency.
More particularly, when such an interferometer is employed in a Fourier transform spectrometer, the exit beam is focused upon a photoconductive detector to produce the electrical interferogram signal. If a sample is so placed that the modulated beam passes through it prior to impinging upon the detector, the analysis performed can determine the absorption spectrum of the sample; other placements of the sample, and other of its characteristics, can be employed and obtained as well.
In those instances in which the optical path length through the interferometer is varied by displacement of a retroreflecting element along the optical axis of the beam, it will be appreciated that the maximum resolution attainable with the instrument is directly related to the maximum path difference that can be produced; i.e., to the maximum longitudinal displacement that is attained by moving the retroreflecting element. Because Michelson interferometers rely upon the wave interference that occurs when the two beams recombine at the beamsplitter, an important factor in determining the quality of such a device is the degree to which the optical elements remain aligned during path-length variation. Accordingly, longitudinal displacement of the moving mirror must be effected with extreme accuracy over the entire range of its travel; i.e., the moving mirror must in most cases remain aligned to within a small fraction of the wavelength of incident light, over a longitudinal displacement of several centimeters. Any irregularity in the mirror guide path, such as might cause even slight tilting of a plane mirror, will lead to significant distortion in the interferogram produced. It is of course well known that substitution of cube-corner and cats-eye retroreflectors for plane mirrors can essentially eliminate such distortion problems; but it is known as well that the use of those elements also entails certain inherent drawbacks.
Precision air bearings and the like have been employed in modern interferometers in an effort to satisfy the extreme accuracy requirements to which movable mirror guides are subject. Such systems are however quite expensive, and moreover they require a supply of pressurized gas for operation. Electronic control systems, with feedback to automatic positioners, have also been utilized as a means for dynamically maintaining mirror alignment; satisfactory self-aligning systems are however difficult to design, and they tend to be relatively large and expensive.
In view of these drawbacks, substantial effort has been devoted to the development of high-quality interferometers that do not rely upon precision bearings or electronic alignment control systems. One approach taken utilizes a tiltable reflector assembly consisting of a pair of parallel, confronting mirrors, rather than a longitudinally displaced retroreflector. As an ostensible improvement thereupon, U.S. Pat. No. 4,915,502, issued on Apr. 10, 1990, teaches a twin-arm interferometer spectrometer having a tiltable reflector assembly by which the optical path lengths of both interferometer beams are varied simultaneously, thereby achieving a desired total path difference by a much smaller rotation of the paired mirrors then would otherwise be required. Such a design affords the advantage of insensitivity to linear movement of the optical element (i.e., only angular displacements will change the path length); moreover, rotating bearings are generally easier and less expensive to produce than are longitudinal or linear ones.
Also of interest is U.S. Pat. No. 4,383,762, issued May 17, 1983, which provides a two-beam interferometer for Fourier spectroscopy in which a rigid pendulum arm is used to mount moving cube corner retroreflectors. The pendulous movement (arcuate oscillation) enables accurate optical path-length variation to be produced in a smoothly guided motion, and the cube corner retroreflectors render the optical system unaffected by the resultant tilt; this avoids the disadvantages for Fourier spectroscopy that are inherent in the deviation from strict linearity that characterizes such pendulous motion.
So-called "porch swing" mounting arrangements are also known in the art, in which structural elements, supported at four pivot points, form a parallelogram by which a mirror, mounted at right angles to one side of the parallelogram, can be translated along an axis that is perpendicular to the mirror's plane. While such designs can provide inexpensive systems in which mounted mirrors are constrained to linear motion, due to the relatively large displacement distances that were deemed necessary they have not herefore been considered practical for use in instruments in which high resolutions are to be realized.
Despite the activity in the art indicated by the foregoing, therefore, a need remains for an interferometer that is compact, stable, and reliable, while also being capable of affording high degrees of resolution and accuracy. Accordingly, it is a broad object of the present invention to provide such an interferometer.
More specific objects of the invention are to provide such an interferometer in which multiple reflections of radiation beams allow large total path length changes to be produced in response to relatively small displacements of movable parts, and which is not subject to the adverse effects of normal vibration, thermal expansion, tilting, and misalignment.
Other objects of the invention are to provide an interferometer having the foregoing features and advantages, which is in addition of relatively incomplex design and construction, and hence is relatively facile and inexpensive to manufacture.
Related objects of the invention are to provide a novel spectrometer that incorporates an interferometer having the foregoing features and advantages, and also to provide a novel radiation amplitude-modulation method that can be carried out quickly and with a high degree of accuracy, utilizing the principles described herein.